docs: def record_diff(old, new):
      """
      Generate a human-readable diff of two performance records.
      """
      return '\n'.join(difflib.ndiff(
          ['%s: %s' % (k, v) for op in old for k, v in op.items()],
          ['%s: %s' % (k, v) for op in new for k, v in op.items()],
      ))